Field of the Invention
The present invention is related to an optical multiplexer which carries out multiplexing and demultiplexing of optical signals in wavelength division multiplexing (WDM) optical communication and the like.
Description of the Prior Art
The structure of a WDM optical communication system is shown in FIG. 1. As shown in FIG. 1, this system includes an optical transmitter 1, optical transmission circuits 2, an optical multiplexer 3, an optical communication path 4, an optical receiver 5, an optical demultiplexer 6, and optical reception circuits 7. In the example shown in FIG. 1, optical signals λ1˜λn having different wavelengths are outputted from respective optical transmission circuits 2 in the optical transmitter 1, and after being multiplexed by the optical multiplexer 3, such optical signals are transmitted to the optical communication path 4. In the optical receiver 5, the optical signals λ1˜λn from the optical communication path 4 are received by respective optical reception circuits 7 after being demultiplexed by the optical demultiplexer 6.
The arrayed waveguide grating (AWG) shown in FIG. 2 is a known prior art optical multiplexer used in WDM optical communication systems. In the AWG of FIG. 2, an input channel optical waveguide 31 for inputting wavelength division multiplexed optical signals, a first slab optical waveguide 32 for horizontally expanding the input light, a channel optical waveguide array 33 constructed from a plurality of optical waveguides having prescribed different lengths, a second slab optical waveguide 34 for creating interference with the light of the arrayed optical waveguides, and an output channel optical waveguide 35 for outputting demultiplexed optical signals are formed on an optical waveguide substrate 30.
In this kind of AWG, in order to obtain a flat demultiplexing spectrum like that shown in FIG. 7 for the pass band, there is a known method of carrying out adjustments so that the electric field amplitude and the electric field phase distribution in the boundary of the channel optical waveguide array 33 and the second slab optical waveguide 34 of FIG. 2 form the amplitude and phase of a sinc function. The sinc function is given below:sinc ξ=(sin ξ)/ξ
In the case where ξ=π(m−149)/60 (for a channel optical waveguide array of 298 optical waveguides), then for the array optical waveguide number m, FIG. 3 shows the electric field amplitude α(m), FIG. 4 shows the electric field phase θ(m)/π, and FIG. 5 shows the electric field distribution. The absolute value of the electric field distribution of FIG. 5 forms the electric field amplitude of FIG. 3, and the positive range and negative range of the electric field distribution of FIG. 5 respectively form the 0 phase and π phase of FIG. 4. When the electric field amplitude and the electric field phase or the electric field distribution in the boundary of the channel optical waveguide array 33 and the second slab optical waveguide 34 of FIG. 2 are established like that shown in FIG. 3˜FIG. 5, the optical distribution in the boundary of the second slab optical waveguide 34 and the output channel optical waveguide 35 in FIG. 2 forms a roughly rectangular distribution as shown by the broken line of FIG. 6. The demultiplexing spectral characteristics of the AWG are given by an overlapping integral of the optical distribution of the broken line of FIG. 6 and the eigen-mode distribution of the output channel optical waveguide 35 of the solid line of FIG. 6. Because the optical distribution is roughly rectangular, the demultiplexing spectral characteristics of the AWG has a roughly flat pass band like that shown in FIG. 7.
As for a method of making the electric field distribution in the boundary of the channel optical waveguide array 33 and the second slab optical waveguide 34 form a sinc function state, there is a known method of using a parabolic optical waveguide like that shown in FIG. 8 for the shape of the input channel optical waveguide 31 at the boundary with the first slab optical waveguide 32. When the parabolic shape and length of the input channel optical waveguide 31 are set at appropriate values, it is possible to obtain a roughly rectangular optical distribution like that shown in FIG. 8. When the rectangular optical distribution of FIG. 8 passes through the first slab optical waveguide 32 and is incident on the channel optical waveguide array 33, the electric field distribution of the channel optical waveguide array 33 forms a sinc function state distribution by a spatial Fourier transform relationship, and a roughly rectangular optical distribution like that shown in FIG. 9 is formed again at the boundary of the second slab optical waveguide 34 and the output channel optical waveguide 35. As a result, a pass band having flat demultiplexing spectral characteristics is obtained in the manner described above.
In this kind of flat-type AWG, there is the problem that it is difficult to obtain flat characteristics when the parabolic shape of the optical waveguide is shifted from the established value, and in order to solve this problem, an example (JP, 11-142661, A) has been proposed in which specific optical waveguides of the arrayed optical waveguides are removed and the luminous intensity distribution is compensated to make the luminous intensity distribution formed on the channel optical waveguide array approach a sinc function state. As for the compensation described above, instead of directly compensating the shift from the established value of the parabolic portion, the far-field thereof and the equivalent luminous intensity distribution in the channel optical waveguide array is compensated to approach a sinc function state, and in this way the distribution spectrum of the AWG is improved to a rectangular state. However, in this kind of prior art flat-type AWG, there is the problem that the AWG itself has a large dispersion, and this problem could not be solved by the prior art compensation method of making the luminous intensity distribution approach a sinc function state.
FIG. 10 shows an example of the dispersion characteristics of a flat-type AWG having a 0.8 nm channel space and a parabolic input channel optical waveguide. The horizontal axis is the relative wavelength from the central channel wavelength. The dispersion value is approximately σ=−20 ps/nm. FIG. 11 shows the results of calculating the pulse waveform distortion generated in the case where a light pulse having a bit rate B=40 Gbps is incident on an AWG having a dispersion of σ=−20 ps/nm. In the case where the AWG itself has a dispersion of |σ|=20 ps/nm, the waveform distortion of the signal is very large, and this is known to increase the error rate of the transmission signal. As is clear from FIG. 11, in the prior art flat-type AWG, the pulse waveform is distorted due to the dispersion in the AWG itself, and this forms a serious problem that makes it impossible to use the AWG as a multiplexer.